1. Field of the Invention
This invention relates to a method of detecting possible porosity zones in the subsurface from seismic velocity data and presenting results on a two-coordinate display indicating spatial positions and depths with a color coding corresponding to the depth interval of the zones.
2. Background of the Invention
The reflection technique of seismic exploration in principle involves the generation of an elastic wave at the near surface which penetrates the earth and is partially reflected back to the surface by the boundaries between successive geologic formations in depth which differ in their density-velocity (acoustic impedance). The reflections are detected by instruments placed on the surface at varying distances from the source of the initial wave. From such detectors the times needed by the wave to travel down through the earth and return after being reflected by each formation boundary at varying depths and possibly having differing dips are recorded, as are any modifications to the waveform.
In practice the seismic waves are initiated at a succession of regularly spaced points (shot points) along the line of survey, and recordings as described are made for each shotpoint. According to such procedure, many of the recording locations are used several times with the source at different locations. Hence this approach is known a "multiple ground coverage" and leads to use of the "CDP" or common-depth-point method developed originally by W. H. Mayne (Common reflection point horizontal data stacking techniques, Geophysics Vol. 27, 1.927-938, 1962). This is an imaging technique which simulates data from a coincident source and detector at the surface using "views" of essentially the same area in the subsurface from different angles as produced by the different source to detector separations. All the recordings in a CDP collection share a common symmetry point at the surface between the source and detector. Profiles of CDP stacked data are usually displayed as vertically plotted wiggle traces along a section with the horizontal axis being the location of the particular synthesized coincident source/detector position and the vertical axis being the two-way travel time of the reflections which are now enhanced by the summing process inherent in the stacking.
The multiple ground coverage view of the subsurface supplied by the survey when combined or stacked according to CDP imaging to form the profile requires the use of stacking velocities. These velocities are derived from an analysis which determines the two parameters which mathematically describe each hyperbolic trajectory corresponding to a reflection event in the trace collections going into the CDP stack. One parameter of the hyperbola is the stacking velocity while the other is the stacking or normal incidence time for the particular event as it will appear on the resulting CDP stacked trace. (See M. T. Taner and F. Koehler, "Digital Computer Deviation and Applications of Velocity Functions", Geophysics, Vol. 34, No. 6, 11-859-881, 1969).
Another relatively common format for displaying seismic data starts with the CDP stacked seismic section which is then further processed to approximate vertical traces of relative or absolute velocity as functions of two-way travel time. Varying colors which relate to the velocities of the geologic formations are then often superimposed to present such information for interpretation. Examples of this kind of display are the SHADCON.TM.* and Seislog.TM.** sections produced on the Applicon ink jet plotter (Applicon, Inc., a subsidiary of Schlumberger, 32 2nd Avenue, Burlington, Mass. 01803). FNT *Trademark Western Geophysical Company of America, A Litton Company FNT **Trademark Teknica Resource Development Ltd., Alberta, Canada
This type of color display also aids in the detection of low velocity zones in the subsurface but does not make direct use of moveout curves in the CDP gathers. Hence this previously available type of display in conjunction with the one described by this invention offer more certainty and precision in the detection and localization of velocity anomalies in the subsurface.